In recent years, the study of nanoparticles has received much interest due to the unique properties of nanoparticles. The physical properties of nanoparticles are fundamentally different from those of the corresponding bulk material. These different physical properties of the nanoparticles are due to the reduced size of the nanoparticles, which is between that of a macroscopic substance and the molecular size. The difference in physical properties between the base material (bulk material) and a nanoparticulate material is due to the increase in the surface/volume ratio and the size of the nanoparticles, which moves toward a magnitude where quantum effects can become dominant. The surface/volume ratio, which increases when the nanoparticles become smaller, leads to an increasing influence of the atoms on the surface of the nanoparticle with respect to the atoms that are located in the interior of the nanoparticle.
The quantum effect phenomenon affects not only the properties of the nanoparticle considered in isolation, but also the properties of the nanoparticle in its interaction with other materials. Therefore, nanoparticles have experienced great interest in areas of research in which a large surface area is required, for example in the field of catalysis, or when used in electrodes, semiconductors, optical devices and fuel cells.
Currently, several methods for the production of nanoparticles exist. These include gas phase deposition, wet chemical synthesis and grinding of the corresponding bulk material.
WO 2009/101091 relates to an apparatus for the production of nanoparticles comprising: at least one module for solution preparation, at least one module for particle synthesis, comprising three chambers heated independently from each other, wherein the at least one module for solution preparation is connected in series with the at least one module for particle synthesis.
This international application also discloses a method for producing nanoparticles, which comprises the following steps: the production of at least two solutions of particle precursors, the separate and substantially simultaneous preheating of the at least two solutions of particle precursors at a first temperature, said first temperature being at least the nucleation temperature of the particles, the mixing of the at least two solutions of particle precursors at a second temperature wherein the second temperature is substantially the same as the first temperature, with the formation of the nanoparticles, the growth of particles at a third temperature, said third temperature being lower than the first temperature.
Example 4 of this application relates to the production of CdSe—CdS core-shell nanoparticles according to this method.
The passivation of CdSe cores or other semiconductor nanoparticles by an inorganic shell material is a known method to increase the stability of the particles and their luminous efficiency, which is measurable in terms of the quantum yield. For example, it is known to coat CdSe nanoparticles with either CdS or ZnS shell material, since the band gap of CdSe is smaller than that of CdS or ZnS, and thus it can be ensured that photo-generated electrons and holes are primarily confined to the core material CdSe. A disadvantage in producing such nanoparticles, which are constructed only of core and shell materials, however, is the lattice strain, arising from the fact that the lattice constants of core and shell materials do not match. This difference in lattice constants is for example 3.9% for CdSe compared with CdS and 12% for CdSe compared with ZnS. This lattice strain may adversely affect the quantum yield and further result in the formation of particles with irregular shape.
Core-shell nanoparticles are mentioned or described in detail for example in the following documents: U.S. Pat. No. 7,144,458 B2 (flow synthesis of quantum dot nanocrystals), US 2003/0017264 A1 (luminescent nanoparticles and methods for their preparation), U.S. Pat. No. 6,207,229 B1 (highly luminescent color-selective materials and methods of making thereof); Peng X., et al. “Epitaxial growth of highly luminescent CdSe/CdS core-shell nanocrystals with photostability and electronic accessibility”, J. Am. Chem. Soc. 1997, 119, 7019-7029; Farmer, S. C. and Patten, T. E., Photoluminescent polymer/quantum dot composite nanoparticles. Chem. Mater. 2001, 13, 3920-3926 concerning core-shell CdS/SiO2 nanoparticles; US 2006/0028882 A1 (Alloyed semiconductor nanocrystals); DE 10131173 A1 (process for producing a core-shell particle, wherein the core is a nanoscale particle); Ping Yang et al. (Highly luminescent CdSe/CdxZn1-xS quantum dots with narrow spectrum and widely tunable wavelength), J. Phys. Chem. C 2011, 115, 14455-14460. Huiguang Zhu et al., Low temperature synthesis of ZnS and CdZnS shells on CdSe quantum dots, Nanotechnology 21 (2010) 255604 teaches that the quantum yield of CdSe core nanocrystals increases from 10 to 36% and from 10 to 40%, respectively, in CdSe/ZnS and CdSe/CdZnS core-shell nanocrystals.
Renguo Xie et al. describe in J. Am. Chem. Soc. 2005, 127, 7480-7488, the synthesis and characterization of highly luminescent multi-layered core-shell nanocrystals having a CdSe core and a CdS/Zn0.5Cd0.5S/ZnS shell. The authors use the so-called SILAR method according to J. J. Li et al. (J. Am. Chem. Soc., 2003, 125, 12567-12575). In a non-continuous process, 3 ml of ODE (octadecene-1) and 1 g of ODA (octadecylamine) were added to a 50-ml reaction vessel and then mixed with the CdSe core particles in hexane and heated to 100° C. The starting materials for the shell material are then added to grow up to 2 monolayers of CdS, 3.5 monolayers of Zn0.5Cd0.5S and two monolayers of ZnS.
The authors argue that because of this gradual change in the composition, the resulting nanoparticles have a high crystallinity. Purportedly, quantum yields of 70-85% for amine-stabilized multi-shell particles in organic solvents and a quantum yield of up to 50% for mercaptopropionic acid-stabilized particles in water could be obtained.
WO 2003/092043 describes luminescent core-shell particles which may have a transition zone which contains an additive which is selected among Cd, Se, Te, S, In, P, As, Pb, O, Si, and Al. Example 1 describes e.g. a preparation method of CdSe/ZnS core-shell particles, in which is added, to the CdSe cores, the additive Cd (as dimethylcadmium) before growing the ZnS shell. WO 2003/092043, however, does not teach how to make a transition zone, which consists only of core and shell material and having opposite gradients for core and shell materials.
If the synthesis of core-shell nanoparticles is carried out according to a batch process, e.g. according to the procedure of Xie et al. or according to WO 2003/092043, one usually notes batch-to-batch variations in product properties, that one wishes to minimize in order to guarantee the buyer of nanoparticles as uniform a product quality as possible.
Both, the method described by Renguo Xie et al. and the method of WO 2003/092043 are not suitable for producing relatively large amounts of core-shell nanoparticles and cannot be carried out continuously. The method according to Xie et al. is also very complicated, since it requires the exact calculation and dosage of the starting materials for the shells.
It would be desirable to provide novel and simply structured core-shell nanoparticles, in which the lattice strain between the desired core material and the desired outer shell material can be reduced in other ways.
A disadvantage of many nanoparticles (NP) syntheses is also the fact that, apart from the growth of the NP, the formation of new NP-nuclei (“nucleation”) occurs. The result are NP dispersions with a very broad size distribution. Especially in the synthesis of core-shell NP, it is particularly important to suppress this unwanted nucleation. Otherwise, there are formed, from the starting materials for the shell material, in addition to the shell, new NP around the core, which can often no longer be separated from the product mixture.
In view of the above-described prior art, it would be desirable to obtain core-shell nanoparticles having a very narrow or narrower particle size distribution.
It is an object of the present invention to provide a novel process for the preparation of core-shell-NP, which suppresses the unwanted nucleation during shell growth.
It is a further object of the present invention to provide a novel process for the preparation of core-shell NP, which leads to a very narrow particle size distribution.
It is a further object of the present invention to provide a novel process for the preparation of core-shell nanoparticles that results, even when the process is carried out repeatedly, in NP having very homogeneous product properties.
It is a further object of the present invention to provide, preferably in a narrow particle size distribution, new core-shell NP, in which the lattice strain between the desired core material and the desired shell material is minimized.
It is a further object of the present invention, to provide core-shell NP having excellent luminescence properties, for example, a very good quantum yield.
Finally, it is an object of the invention to provide a tubular reactor and its use for the continuous production of core-shell nanoparticles, which allows for a technologically and economically optimal implementation of the above process while achieving the advantages of the present invention.